Recently, organic transistor elements, in which an organic semiconductor has been applied, have attracted attention (refer to Advanced Materials, 2002, Vol. 14, p. 99). Such organic transistor elements can be formed by a wet process such as a printing method, and thus the production cost can be significantly reduced as compared with existing silicon processes, which require expensive production facilities. In addition, since the process can be performed at low temperatures, the organic transistor elements can be formed even on a plastic substrate. Thus, organic transistor elements have been actively studied as a technology for realizing flexible thin-film transistor (TFT) elements. As application examples of organic TFTs, examination of electronic paper, displays, radio-frequency identification (RFID), and various sensors has been conducted toward realization of practical use.
An organic transistor is an organic semiconductor element including, as essential components, three constituents, namely, an electrode layer, an insulating layer, and a semiconductor layer. Methods for forming the electrode layer, the insulating layer, and the semiconductor layer that constitute the element are divided into a wet process such as a printing method and a dry process such as vacuum deposition or sputtering. From the standpoint of productivity and a reduction in the cost, the wet process is overwhelmingly advantageous (refer to Chemistry of Materials, 2004, Vol. 16, p. 4543).
In the formation of transistor circuits, patterning of an organic semiconductor is important. It is necessary to form a semiconductor layer in a desired region of the transistor circuits, for example, in the case of a lateral TFT element, it is necessary to form a semiconductor layer so that the semiconductor layer extends over each source electrode and a corresponding drain electrode, and so that the semiconductor layer is electrically isolated from adjacent element portions. This structure can suppress interference between adjacent elements due to a leakage current, and thus it is possible to reduce an off-state current of a transistor and to accurately control the circuits.
As a method for forming an organic semiconductor layer by a wet process, a spin-coating method and a dip-coating method are most generally employed. However, in these methods, the organic semiconductor layer is formed as a solid film that entirely covers a plurality of device regions. It is difficult to form a desired organic semiconductor pattern only in a necessary region using these methods. An ink jet method has been studied as a method for forming an organic semiconductor layer at a desired position on transistor circuits. However, it is difficult to form a pattern having a size of 50 μm or less using the ink jet method, and thus the pattern fineness is limited. In addition, it is difficult to form a desired shape. Furthermore, the printing speed is low, and thus there is a problem in terms of productivity.
In order to solve these problems, recently, various pattern transfer methods using a liquid-repellent transfer plate have been studied. For example, Japanese Unexamined Patent Application Publication No. 2007-67390 (“JP '390”) and Japanese Unexamined Patent Application Publication No. 2007-184437 (“JP '437”) disclose a method for producing a semiconductor device (element) and an apparatus for producing a semiconductor device (element) characterized in that an organic semiconductor crystal layer is formed on a water-repellent surface of a stamp substrate having water repellency, and the stamp plate is pressed on a transfer-receiving substrate on which source and drain electrodes have been formed in advance to transfer the semiconductor layer. However, neither JP '390 nor JP '437 discloses a composition of a semiconductor ink used in the printing method using such a transfer plate (stamp substrate) except that an organic semiconductor is simply dissolved in an organic solvent, and the resulting solution is provided as the ink. Even when a solution prepared by dissolving an organic semiconductor is merely used, it is difficult to form a precise and fine pattern on the stamp substrate without defects, and furthermore, it is difficult to completely transfer the pattern to the transfer-receiving substrate. A large number of pattern defects are generated, and thus such a solution cannot be used in practical applications. A special organic semiconductor ink that is the most suitable for the above printing method has been desired.
The following printing properties are required for an organic semiconductor ink for transfer, the ink being used for obtaining a desired pattern by a method, such as a reverse printing method or a microcontact printing method, in which an ink layer is formed on a liquid-repellent transfer substrate, patterning of the ink is performed as required, and the ink layer is then transferred to a printing base material, for example: (1) A uniform ink coating film can be formed on a surface of a liquid-repellent transfer substrate. (2) A dry ink film or a semi-dry ink film is easily transferred from the transfer substrate to a transfer-receiving base material on which an organic transistor is to be formed. Furthermore, in the case where the reverse printing method is applied, (3) an accurate pattern can be formed using a removal plate from an ink solid coating film formed on the transfer substrate. In addition to these printing properties, the organic semiconductor ink requires that a formed organic semiconductor thin film have excellent transistor characteristics.